1. Field of the Invention
The invention relates to a method for fabricating a metal gate transistor and a polysilicon resistor and also relates to a polysilicon resistor structure.
2. Description of the Prior Art
In the field of semiconductor fabrication, the use of polysilicon material is diverse. Having a strong resistance to heat, polysilicon materials are commonly used to fabricate gate electrodes for metal-oxide semiconductor transistors. Devices fabricated by polysilicon still have many drawbacks, however. In contrast to most metal, polysilicon gates are fabricated by semiconductor materials having high resistance, which causes the polysilicon gate to work under a much lower rate than other metal gates. In order to compensate for this slightly lowered rate of performance, a significant amount of silicides is applied during the fabrication of polysilicon processes, such that the performance of the device can be increased to an acceptable level.
Furthermore, gate electrodes fabricated by polysilicon also result in a depletion effect. A depletion region at the interface between the gate and the gate dielectric layer will occur during operation. This depletion region not only thickens the gate dielectric layer, but also lowers the capacitance of the gate, and ultimately reduces the driving ability of the device. In order to solve this problem, metal gates are used to replace conventional polysilicon to fabricate gate electrodes for MOS transistors.
The conventional technique for fabricating a metal gate transistor includes firstly forming a dummy gate by polysilicon material. Then, after the MOS transistor is completed, the dummy gate will be removed by an etching process. Later, a metal layer is deposited in the region where the dummy gate was originally located.
In the semiconductor field, polysilicon is often used to form resistors. In order to integrate the fabricating process, the polysilicon resistor and the transistor are formed simultaneously by the same fabricating steps. Traditionally, the high resistance structure of the polysilicon resistor is formed together with the polysilicon gate of the transistor. Currently, in order to form metal gates, the polysilicon gate serving as a dummy gate is removed. The high resistance structure of the polysilicon resistor will also be removed along with the dummy gate. Later, a metal layer is filled into the region where the dummy gate was originally located and the region where the high resistance structure was originally located. The metal layer that filling the region where the high resistance structure was originally located will deteriorate the performance of the polysilicon resistor.
Therefore, an integrated method for fabricating the metal gate transistor and the polysilicon resistor is needed to overcome the above-mentioned drawbacks.